In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for writing an integrated circuit comprising such a fine pattern. However, the conventional exposure techniques using light exposure have been close to the limit of the conventional photolithography method while semiconductor devices have had finer patterns at an accelerated pace. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, has been considered as being promising as the exposure technique for 45 nm or below. In this Description, it should be noted that the phrase “EUV light” means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
It is impossible to use EUV light in conventional dioptric systems as in photolithography using visible light or ultraviolet light since EUV light is apt to be absorbed by any substances and since the refractive index is of the substances is close to 1 at the above wavelength. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a stacked member to be used for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, formed thereon in this order. The reflective layer normally comprises a reflective multilayer film, which comprises high-refractive layers and low-refractive layers alternately stacked to increase a light reflectance when irradiating a film surface with EUV light. The absorber layer comprises a material having a high absorption coefficient in connection with EUV light, specifically, for example, a material containing Cr or Ta as the main component.
Patent Document 1 discloses a nitride of a tantalum/boron alloy (TaBN), an oxide of a tantalum/boron alloy (TaBO) and an oxynitride of a tantalum/boron alloy (TaBNO) which have a high absorption coefficient to EUV light and have a low reflectance to deep ultraviolet light in the wavelength range (190 nm to 260 nm) of pattern inspection light, as preferred materials of an absorber layer.
Further, Patent Document 1 discloses that the absorber layer preferably has an amorphous structure in order that the absorber layer has a surface excellent in smoothness, and in order that a TaBN film, a TaBO film and a TaBNO film have an amorphous structure, the B content in these films is preferably from 5 to 25 at %.
Patent Document 1: JP-A-2004-6799